Input circuit for intercommunication system



May 21, 1963 A. LIBERMAN INPUT CIRCUIT FOR INTERCOMMUNICATION SYSTEMOriginal Filed May 51, 1955 Illl llllll-lllv ,Irnfanior'.

- firie A z'er'ma m W M W Patented May 21, 1953 3,090,834 INPUT CIRCUITFOR INTERCOIVEMUPHCATiON SYSTEM Arie Liherman, Talk-A-Phone, 1512 S.Pulaski Road, Chicago, Ill.

Original application May 31, 1955, Ser. No. 512,206, new Patent No.2,980,767, dated Apr. 18, 1961. Divided and this application Oct. 23,1956, Ser. No. 617,713

6 Claims. (Cl. 179-25) This invention is concerned with a carrier waveintercommunication system which utilizes power lines for transmitting amodulated carrier wave between stations, and more particularly with aninput circuit for such a system. This application is a division of mycopending application Serial No. 512,206, filed May 31, 1955, now Patent2,980,767, issued April 18, 1961.

It is a principal object of this invention to provide a new and improvedintercommunication station, particularly adapted for use inmulti-channel systems.

One feature of the invention is the provision of an arrangement forcoupling a station to the power line, comprising a tuned couplingcircuit for the station, an impedance and means for connecting thecoupling circuit and the impedance across the line, whereby the effecton the tuned circuit of reactive loads added to the line is minimized.Another feature is that the station has a tuned line couplingtransformer and a radio frequency choke is connected in series with aWinding of the transformer, across the line, whereby the effect on thetuning of said transformer of reactive loads connected to the line isminimized.

Further features and advantages will readily be apparent from thefollowing specification and from the drawing which is a schematicrepresentation of the circuit.

The present system is particularly designed for multichannel use ascontrasted with single channel systems, such as that shown in mycopending application, Serial No. 430,956, filed May 19, 1954, nowPatent 2,887,533, issued May 19, 1959. The general principles ofoperation of the present system are quite similar to those of the twostation system shown in the copending application and reference may behad thereto for details of the external physical appearance of thestation units and of the interconnection and operation thereof.Preliminarily, one of the biggest difierences between the two systems isthe provision in a multi-channel system, of a plurality of selectablecircuit components for tuning each station to one of a group ofdifferent frequencies.

While one specific circuit is shown in the drawing and values will begiven herein for the components thereof, it is to be understood thatthis circuit is intended to be representative only and manymodifications will be readily apparent to those skilled in the art.

The circuit which will be described is that of a master station whichcan originate calls to and answer calls from any other station, ascontrasted with a staff station which can only answer calls, ororiginate calls to other stations on its frequency. In the embodimentshown the station is provided with six sets of circuit components byvirtue of which it may operate on any one of six different frequencies,the operating frequency being selected through manipulation of selectorswitches A, B, C, I), E and F. In contrast, a staif station for use inthe same system operates on a single, fixed frequency. Many of thecomponents of the circuit serve a dual function, operating in one mannerduring transmission and in another manner during reception and theinterconnection of the various circuit elements, and thus the manner inwhich they operate, is determined by the position of an eight-poledouble-throw switch 10, which will hereinafter sometimes be referred toas the talk-listen" switch. This switch is shown in the listen positionin the drawing and in practice, is generally spring biased to thisposition.

Turning now more specifically to the drawing, it will be seen that powercord 11 is provided with a male connector 12 which may be inserted intoa suitable electrical outlet of a 110 volt A.C. or DC power system. Thepower line (not shown) then serves not only to supply power to the unit,but also to carry the modulated signals between the various units of thesystem. Conductor 11a of the power cord is connected to a common orground line 13. Conductor 11b is broken by On-01f switch 14.

When switch 14 is closed, the series connected filaments 15 of thevarious tubes in the unit are connected across the power line as is anindicator light 16 which is lighted when the power is on. The powersupply section of the unit comprises a single, half-wave rectifier 17, a35W4, the anode of which is connected through resistor 18, 47 ohms, toone side 11b of the power line. A capacity input filter comprisingcapacitor 19, 40 f. (microfarads), resistor 20, 220 ohms, capacitor 21,20 i, resistor 22, 2200 ohms and capacitor 23, 8 ,uf., is connectedbetween the cathode of rectifier 17 and the common line 13. The powersupply has an output voltage of -95 volts depending on the currentdrawn. A high B+ of volts is available across capacitor 21.

The signal input circuit for the unit is also connected, throughblocking capacitor 28, .1 pi, and section 10a of the talk-listen switch,across the power line. The input circuit includes primary winding 29::of antenna transformer 29 connected in series with a radio frequencychoke 30. Antenna transformer secondary coil 2% is connected throughsection 10b of the talk-listen switch to ground and is tuned toresonance at the desired frequency by one of the variable capacitorsections 31a, 31b, 31c, 31d, 31c or 31 depending on which of the stationselector switches A, B, C, D, E and F is chosen, it being necessary thatone of the selector switches be actuated for the unit to operate.

The antenna transformer has a primary winding of 80 turns of 7 strandNo. 40 Litz wire with an inductance of ,uh. (microhenry) and a secondarywinding of 335 turns of 7 strand No. 40 Litz wire, 2.8 mh. The values ofthe various capacitors depend on the frequencies used, which in theparticular unit being described are as follows:

The incoming signal is coupled through a circuit, including capacitors35, 1.2 n f. (micromicrofarads), and 36, 0.27 ,uf., and which will bedescribed more fully later, to the tuned grid circuit of the radiofrequency amplifier stage 37, a SOCS. The tuned grid circuit includeswinding 38a (a part of oscillator transformer 38) and the selected oneof the variable capacitors 32a, 32b, 32c, 32d, 32e and 32). The cathodeof radio frequency amplifier 37 is returned to the common connection orground 13 through a bias network including resistor 40, 330 ohms andcapacitor 41, .1 ,uf. The screen grid of the amplifier is connected atthe junction point of two 47,000 ohm resistors 42 and 43 connectedacross the high voltage supply. Grounded resistor 42 is bypassed bycapacitor 44, .01 ,uf. The anode of amplifier 37 is connected throughsection 100 of the talk-listen switch and load resistor 45, 3600 ohms,to the power supply.

The amplified signal from tube 37 is coupled through capacitor 46, 500uni, to detector circuit including crystal diode 47, 1N48, and a loadincluding resistor 48, 270,000 ohms and two shunt filter capacitors 49and 50, 500 ,uuf. each. The audio signal is developed across volumecontrol resistor 55, 500,000 ohms, and is coupled through section 10d ofthe talk-listen switch and the series.

combination of resistor 56, 470,000 ohms, and capacitor 57, .005 ,uptfi,to the control grid of audio amplifier 58, a 12AU6, which is returned toground through resistor 59, 10 megohms, shunted by carrier by-passcapacitor 60, 500 .Luf. The anode and screen grid of tube 58 areconnected, respectively, through resistors 61, 470,000,

ohms, and 62, 1.5 megohms, and a common load resistor 63, .5 megohm, tothe power supply. The purpose of resistor 63 will become apparent laterin the discussion of the squelch circuit.

The output oftube 58 is developed across resistor 61 and is coupledthrough capacitor 64,, .005 ,uf., to the control grid of power amplifier'65, a OC5. The control grid of amplifier 65 is returned to groundthroughresistor 66, 470,000 ohms, while the cathode thereof is connectedto ground through bias resistor 67, 220 ohms, shunted :by capacitor 68,,uf.; the control grid and cathode are connected together by a capacitor69, 100 ,u tf. The anode of audio output amplifier 65 is coupled throughprimary winding 70a of audio output transformer 70 to a high B+(approximately 100 volts) at the inne tion of resistors and 22; thescreen grid being connected directly to this junction. Secondary winding70b of the output transformer is connected through section 102 of thetalk-listen switch to loud speaker '71.

'An automatic gain control voltageis developed acrosscapacitor 75, .01,uf., which is connected through resistor 76, 1.5 megohms, to thenegative terminal of the detector circuit. her 37 is returned throughsection 10 of the talklisten switch. to the juncture between capacitor75 and resistor 76; the other terminal .of capacitor 75 being groundedthrough section 10g of the talk-listen switch. Capacitor 75 is shuntedby resistor 77, 4.7 megohms.

Inasmuch as the unit when in operation is normally left in listen?condition, a squelch circuit is provided for preventingnoise appearing,on the powerline from being amplified .and reproduced when no signal isbeing received. The basic elements of the squelch circuit is squelchtube 80, a 12AU6, connected for tetrode operation with the suppressorgrid tied to the anode, which is in turn connected through resistor 63to the power supply. The screen grid of squelch tube 80 is connecteddirectly to the power supply and the cathode is returned to a variablepositive voltage (0-4 volts) on" a voltage divider made up of fixedresistor 81, 47,000 ohms and potentiometer 82, 2,000 ohms. The controlgrid of the squelch tube is connected to an integrating circuit made upof resistor 83, 1.5 megohms and capaci tor 84, .01 ,uf., connected tothe negative terminal of the detector. .Withthe talk-listen switch inthe listen position, andwith no signal being received, the .control'grid of the. squelch tube will have a voltage of Zero or just slightlynegative applied thereto. Tube 80 will conduct heavily since the screengrid has a high positive voltage (80 volts), although the high platecurrent flowingthrough resistor 63 will reduce the anode voltagesubstantially to zero.

Since both the screen grid and anode of audio amplifier 58 are connectedto the anode of squelch tube 80, the audio amplifier itself will be cutoff. Sensitivity control 82 may be adjusted so that. this situationobtains regardless of the normal amount of noise in the noiseencountered on the line. As soon as a signal is received by the station,the voltage applied to the control grid of the squelch tube will becomeincreasingly negative, cutting this tube off and permitting the anodeand screen grid voltage of audio amplifier 58 to rise to The controlgrid of radio frequency ampli-- an operable value. Since the squelchtube operates on both the screen grid and anode of the amplifier,distortion of the incoming signal by partial cut off of the squelch tubeis minimized. The squelch circuit is eX- tremely sensitive, and whenproperly adjusted may be triggered 'by a rectified signal as small as0.10 volt on the grid of the squelch tube. The long time constant of theintegrator circuit (RC=.O15 sec.) connected to the grid of squelch tube80 delays the build-up of negative voltage on the grid of the squelchand prevents cut-off of the squelch tube by bursts of high amplitudenoise energy.

Turning back now to the input circuit of radio frequency amplifier 37,it will be recalled that primary coil 29a of tuned antenna transformer29 is connected in series with coil 30 across the power line.Multi-channel carrier wave intercommunication systems which utilize thepower lines for communicating messages between units must necessarilyutilize carrier waves of different frequencies if more than oneconversation is to be carried on at a time. This in turn requires theuse of tuned, frequency-sensitive circuits in the input of the amplifierunit. This in itself is not a difiicult problem, but it often happensthat, after the system has been installed, additional reactive loads maybe connected to the power line in such a manner that they afiect thetuned input circuit, to the point of detuning it far enough so that ithas little or no response at the desired frequency. An example of such aload which might be connected to the line is a small A.C.-D.C. radioreceiver which normally has an .05 ,uf. capacitor connected across thepower supply in put; a sufficient capacity to detune the input circuitof the intercommunication station system seriously. The radio frequencychoke 30, which is connected in series with antenna transformer primarycoil 29a across the line, comprises 80 turns of No. 28 wire and has aninductance of 38 [Lil It has been found that with this additionalinductive impedance in series with the amplifier input circuit thedetuning effect of additional capacitive loads connected to the powerline is negligible;

It is desirable that .the selectivity and sensitivity characteristics ofthe tuned input circuits of the station be approximately the same forall channels. Since the system operates on frequencies from 98 to 250kilo-.

cycles, it was found necessary to provide a special coupoint andreturned through capacitor 36 and talk-listen,

switch section 10g to ground. Capacitor 35 together with the distributedcapacity of the circuirtcomponents (a total of about 5.5. ,u tfi},provide adequate coupling of the signal at the higher frequencies whilecapacitor 36 insures adequate coupling on the low frequency bands, thecircuits being slightly over-coupled on hands A and B to achieve thenecessary band width.

When the talk-listen switch is pressed, moving it .to, talk position,the system is converted from a receiver to a transmitter, by changingthe operation of some of the circuit elements. Amplifier tube 37 isutilized, in

the talk position, as a modulated oscillator with a tuned grid circuitincluding coil 38a, compensating coil 85, compensating capacitor 86,variable up to 30 ,u rf., and the selected one of the tuning capacitorsin group 32. Feedback is provided from the plate circuit to the gridcircuit through coil 38b which is connected to the anode of the tubethrough section 100 of the talk-listen switch; the plate circuit of theoscillator being connected to the high B+ connection of the powersupply. Self-bias of the oscillator is provided by a grid leak biasarrangement including resistor 88, 270,000 ohms, and capacitor 87, 100 nf. The A60 system is shorted during transmission by section g of thetalk-listen switch. The output of the modulated oscillator isinductively coupled from winding 38a of the oscillator coil to linkwinding 380 which is connected through section 10a of the talklistenswitch to the power line and returned to the common terminal throughchoke 30. Again, choke 30 minimizes the effect of reactive loads'on theline, which during transmission would have the effect of detuning theoscillator. The oscillator coil windings are all of 7 strand, No. 40Litz wire, winding 380 being 450 turns, 2.7 mh.; winding 38b, 144 turns,2.0 mh. (slug tuned); and winding 38c, 80 turns 150 uh.

The modulation information is coupled from transducer 71 (the loudspeaker is used also as a microphone) through section 10e of thetalk-listen switch to an audio input transformer 90, the secondary ofwhich is connected through section 10d of the talk-listen switch to thecontrol grid of audio amplifier 58. (The continuous oscillation producedby tube 37 is rectified in detector 47 using a sufiicient negativevoltage on the grid of the squelch tube 80 to cut this tube off topermit operation of audio tube 58 during transmission.) The audio signalis coupled from audio amplifier 58 to modulator tube 65 where it isfurther amplified and coupled through capacitor 91, .05 ,uf., andsection 10h of the talk-listen switch to the screen grid of oscillator37. Section 10h of the talk-listen switch also connects resistor 92,15,000 ohms, in parallel with resistor 42 of screen grid Voltage dividerduring transmission. Thus, a relatively low voltage, of the order ofvolts, is applied to the oscillator screen grid during transmission.This low voltage keeps the stage in oscillation but at a low amplitudeexcept during the positive swing of the modulating signal. This circuitand its noise reducing operation are fully described in myaforementioned copending application and reference may be had theretofor further details thereof.

It will be recalled that the selected capacitors of group 32 areutilized both during reception and transmission to resonate with winding38a, tuning the grid circuit of tube 37 to the same frequency. However,as the input impedance of tube 37 (which is in shunt with the tunedcircuit) varies between the two conditions of the tube, the tunedcircuit made up of winding 38a and the selected one of the capacitors32, will not resonate at the same frequency during both reception andtransmission. In order to correct this condition and to insure that theunit will operate at substantially the same frequency during bothtransmission and reception, on each band, a compensating circuitincluding compensating circuit including compensating coil 85 andcompensating capacitor 86 are added to the circuit. As shown in thedrawing, coil 85 is placed in series with winding 38a of the oscillatortransformer while capacitor 86 is placed in parallel with the seriescombination mereof, through operation of section 10 of the talklistenswitch. In the embodiment which is discussed herein, compensating coil85, 250 ;/.h has 150 turns of 7 strand No. 40 Litz wire and is slugtuned, while capacitor 86 is variable with a maximum capacity of 30,uuf.

In adjusting the tuned circuits, capacitors 31 and 32 are first adjustedto the proper frequencies with the unit operating as a receiver. At thesame time the antenna and oscillator coils are properly adjusted on thelowest frequency band. The unit is then operated as a transmitter andcompensating coil 85 is adjusted on hand A to cause operation at theproper frequency; the compensating capacitor 86 is similarly adjusted onband F. The above steps or part of them may then be repeated one of moretimes to eliminate any detuning due to interaction between the variouscircuits.

While I have shown and described certain embodimerits of my invention,it is to be understood that it is cap-able of many modifications.Changes therefore, in the construction and arrangement may be madewithout depanting from the spirit and scope of the invention asdisclosed in the appended claims.

I claim:

1. In a carrier intercommunication system which includes a plurality ofstations and a power line interconnecting the stations and to whichother loads may be connected, coupling means of the character describedfor connecting one of said stations to the line, comprising: a linecoupling circuit for said station tuned to the carrier frequency; achoke having an impedance at carrier frequency, less than the impedanceof said coupling circuit; and means for connecting said coupling circuitand said choke in series across said line, whereby the effect on saidtuned circuit of capacitive loads added to said line is minimized.

2. In a carrier intercomrnunication system which includes a plurality ofstations and a power line interconnecting the stations and to whichother loads may be connected, coupling means of the character describedfor connecting a station to the line, comprising: an input circuit forsaid station tuned to the carrier frequency; a reactive impedance havinga value less than the input impedance of said tuned input circuit; andmeans connecting said input circuit and said impedance across said line,whereby the efiect on said tuned circuit of reactive loads opposite incharacter to said reactive impedance added to said line is minimized.

3. In a carrier intercommunication system which includes a plurality ofstations and a power line interconnecting the stations and to whichother loads of a reactive nature may be connected, coupling means of thecharacter described for connecting one of said stations to the line,comprising: an input circuit for said station including a transformerhaving a winding and tuned to the carrier frequency; a radio frequencychoke connected in series with said winding having an impedance atcarrier frequency, less than the impedance of said tuned transformer;and means for connecting the series combination of said winding andchoke across said power line, whereby the effect on said tuned circuitof capacitive loads added to said line is minimized.

4. In a carrier intercommunication system which includes a plurality ofstations and a power line interconnecting the stations and to whichother loads of reactive nature may be connected, coupling means of thecharacter described for connecting a station to the line, comprising: aninput circuit for said station including a transformer having a windingand tuned to the carrier frequency; a radio frequency choke connected inseries to said winding, said choke having an inductance substantiallyless than the inductance of said winding; and means for connecting theseries combination of said winding and choke across said power line,whereby the effect on said tuned circuit of capacitive loads added tosaid line is minimized.

5. In a carrier intercommunica-tion system which includes a plurality ofstations and a power line interconnecting the stations and to whichother loads of a reactive nature may be connected, coupling means of thecharacter described for connecting one of said stations to the line,comprising: an input circuit for said station including a transformerhaving a primary winding and a secondary winding tuned to the carrierfrequency; a radio frequency choke connected in series with said primarywinding having an impedance at carrier frequency, less than theimpedance of said input circuit; and means for connecting the seriescombination of said primary winding and choke across said power linewhereby the effect on said tuned circuit of capacitive loads added tosaid line is minimized.

6. In a carrier intercommunication system which includes a plurality ofstations and a power line intercon- 74 necting the stations and to whichother loads of a reactive nature may be connected, coupling means of thecharacter described for connecting a station to" the line, comprising:an input circuit for said station including a transformer having aprimary Winding and a secondary winding tuned to the carrier frequency;a radio frequency choke connected in series with said primary windingand having an inductance of the order of one-quarter of the inductanceof the primary winding of said transformer and an impedancesubstantially less than the input impedance of said input circuit; andmeans for connecting the series combination of said primary winding andchoke across said power line whereby the eifeot on said tuned circuit ofcapactive 1oads added to said line is minimized.

References Cited in the file of this patent UNITED STATES PATENTS980,358 Squier Jan. 3, 1911 1,862,137 Curtis June 7, 1932 2,143,563 Levyet a1 Jan. 10, 1939 2,632,812 Cooney Mar. 24, 1953 2,837,599 Marks June3, 1958 V

1. IN A CARRIER INTERCOMMUNICATION SYSTEM WHICH INCLUDES A PLURALITY OF STATON AND A POWER LINE INTERCONNECTING THE STATIONS AND TO WHICH OTHER LOADS MAY BE CONNECTED, COUPLING MEANS OF THE CHARACTER DESCRIBED FOR CONNECTING ONE OF SAID STATION TO THE LINE, COMPRISING: A LINE COUPLING CIRCUIT FOR SAID STATION TURNED TO THE CARRIER FREQUENCY; A CHOKE HAVING AN IMPEDANCE AT CARRIER FREQUENCY, LESS THAN THE IMPEDANCE OF SAID COUPLING CIRCUIT; AND MEANS FOR CONNECTING SAID COUPLING CIRCUIT AND SAID CHOKE IN SERIES ACROSS SAID LINE, WHEREBY THE EFFECT 